Geochemical prospecting method



Fzp mi xP. 39524934-5 Aug. 18, 1970 5. w, M DT 3,524,345

GEOCHEMICAL PROSPEGTING METHOD Filed July 15, 1968 2 Sheets-Sheet 1 :73 E IO 8 P 3 6 E a 4 0 (I) 5 2 a: D: o o o: I .8 .6 2 Z .4 E o 0: 2

I l 0 50 I00 200 300 SALINITY, gm./l.

FIG. I GENE W. SCHMIDT INVENTOR.

BY ATTORNEY 18, 1970 G. w. scI-IMIDT 3,524,346

GEOCHEMICAL PROSPECTING METHOD Filed July 15, 1968 2 Sheets-Sheet 2 CONTAINER FULL AND TIGHTLY CAPPED L6 *2? a R fi v m CONTAINER ULL 2 NOT TIGHTLY CAPPED 3I.2-- O I- C 3 I: CONTAINER PARTIALLY FULL 0 Lo AND TIGHTLY CAPPED 1% Z .6 U E CONTAINER PARTIALLY FULL-NOT TIGHTLY CAPPED O I v T 5 $-A--- o I 2 3 4 5 6 7 8 TIME (DAYS) KEY FIG. 2 A BENZENE GENE w. SCHMIDT o TOLUENE INVENTOR.

BY PM ATTORNEY United States Patent O US. Cl. 73-153 8 Claims ABSTRACT OF THE DISCLOSURE The concentration of at least one aromatic hydrocarbon is determined in a sample of formation water from a reservoir formation in the earth. The measured value is compared with the so-called target value for this hydrocarbon. If they are nearly alike, the point from which the sample was obtained is close to a reservoir of crude oil. Greater differences between these two values represent greater distance to the crude oil accumulation The target value is determined by contacting the type of crude oil to be expected in the reservoir from which the water sample came with water solutions of salt and measuring the concentration of the aromatic hydrocarbon in the solutions. This permits determining the variation in concentration of this aromatic hydrocarbon as a function of the salinity of the dissolving water. The salinity of the sample of the water from the well is determined, It is then possible to determine the target value, i.e., the value of concentration of this aromatic hydrocarbon, which would exist at the point of contact between crude oil and formation water of that salinity in the subsurface reservoir.

FIELD OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART In the oil industry wildcat wells are drilled in an attempt to tap unknown subsurface hydrocarbon reservoirs. There is insufiicient surface evidence in most cases to assure that the first well drilled in a new location is likely to find crude oil. Experience ordinarily available prior to drilling such a well indicates the formations in which it is expected crude oil may be found, and (from other fields draining such formations) the characteristics of the crude oil that would be produced. If such a well is drilled and a test is made of the fluids produced, it is frequently found that nothing is recovered but formation water. It is very desirable to learn from such a water sample as to whether the well is probably near a crude oil reservoir or whether it is unlikely that such reservoir is anywhere near the well drilled Coggeshall and Hanson in their US. Pat. 2,767,320 have pointed out that there are water-soluble constituents in a large number of crude oils, ordinarily the aromatic hydrocarbons benzene and toluene. These constituents dissolve into the surrounding formation water and by ordinary diffusion are driven to some distance from the oil, the concentration diminishing with distance from the accumulation. Such dissolved hydrocarbons generally are confined to the zone in which they are associated with the oil, that is, they do not ordinarily migrate vertically across impermeable barriers. Accordingly, Coggeshall and Hanson taught making measurements of the concentration of benzene in two wells. It is taught that one can determine from these concentrations the direction in which one should drill a third well, i.e., in the direction of increasing concentration.

One major difliculty with this situation is that frequently one wishes to obtain information as to whether or not it ts likely that there is a crude oil accumulation in the vicinity of the dry hole in the complete absence 3,524,346 Patented Aug. 18, 1970 of a second well. By the method which is taught herein, it is possible to make such a determination before a sec ond well has been drilled Two papers by W. M Zarella and others assist in the technical understanding of the nature of the diffusion of hydrocarbons in subsurface brines. These are Analysis and Interpretation of Hydrocarbons in Subsurface Brines, Division of Petroleum Chemistry of the American Chemi cal Society, Los Angeles Meeting, Mar. 31-Apr. 5 (1963), and Analysis and Significance of Hydrocarbons in Subsurface Brines, Geochimica et Cosmochimica Acta, vol. 31, 1155-66 (1967) SUMMARY OF THE INVENTION At least one aromatic hydrocarbon (preferably ben zene or toluene) is measured in a sample of formation water taken from the stratum under investigation in a well, The crude oil of the kind or type known to occur in such formation is contacted with water solutions of salt of at least two different concentrations for a suflicient time to permit the concentration of the hydrocarbon on which the concentration was measured to reach at least approximately its equilibrium value After this, the concentration of such hydrocarbon is determined in each of the salt solutions, by means of which the variation of such con= centration can be determined in terms of the salinity of the water in contact with the crude oil. The salinity of the water sample from the well is determined, or at least a quantity directly related to this salinity (such as re= sistivity, chloride ion concentration, or the like) is measured, so that it is possible to know what the total salinity is in the formation water. From this, and the knowledge of the variation of concentration of the hydrocarbon in terms of salinity of water solutions, the so-called target value is computed, which is the concentration of the hydrocarbon to be expected if the formation water sample had been in contact with the crude oil in the immediate vicinity of the dry hole. Then from the difference between the target value and the value of the hydrocarbon concentration actually determined, a qualitative answer can be given as to whether the well has been drilled near a reser= voir of the crude oil of the type to be expected in that formation,

DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENTS As has been already mentioned, the problem involved is typified by the circumstance that a wildcat well has been drilled to a reservoir formation of a type known frequently to be oil producing, but the drill stem test of this formation has only shown the presence of formation water. One Wishes to determine whether or not this Well has been drilled near an accumulation of crude oil of the type to be expected. It is known that frequently crude oils contain somewhat water-soluble constituents. Those of chief interest are the aromatic hydrocarbons, benzene and toluene. Other aromatic hydrocarbons may be present, but usually in much smaller amounts. Such water-soluble aromatic hydrocarbons will diffuse from an. oil or wet-gas accumulation into the surrounding aquifer, with a concentration which diminishes with distance from the accumulation. Ordinarily, they are confined to the stratum in which the oil exists, since they cannot migrate vertically across the impermeable barriers defining the top and bottom of the reservoir. In this zone their concentration is affected not only by the concentration in the crude oil but by the fact that the total dissolved solids in the formation water has a major effect on the maximum possible concentration of such hydrocarbons, they can be sorbed on the rocks forming the reservoir, and they can be affected by the subsurface flow of formation water through the reservoir rocks. In any event, if the maximum concentrataion of the aromatic hydrocarbon in the formation Water can be determined and compared to the equivalent concentration of hydrocarbon actually present in the sample of formation Water taken from the well, it can be determined whether there is no crude oil present near the well, whether the Well is close to such accumulation, or whether it lies in an intermediate zone spaced some distance from; the oil.

I have found how to determine the maximum concen tration of such hydrocarbon that can be expected, which is referred to in this specification as the target value. Thus, if this target value is, for example, 1 milligram per liter, and the concentration of this hydrocarbon in the connate water from the well is 0.6 milligram per liter, it is apparent that the well is near but not essentially in contact with the crude oil accumulation. Experience has demon-= strated that this method is generally effective up to about three or four miles, that is, that benzene and toluene diffuse outward in the water that far from the edge of a petroleum accumulation, provided that water communication exists over this distance. Because of this diffusion characteristic, this method actually enlarges an exploration target. For example, an undiscovered oil accumulation of four square miles can produce an exploration target in this method of the order of 70 square miles. The benzene or toluene found in significant concentration in this outer area indicates that there is this undiscovered accumulation a few miles away at most. Further working of geological, geophysical and geochemical data can then be employed to locate a subsequent well to tap this accumulation.

The determination of the concentration of an at least partly water-soluble aromatic hydrocarbon, such as benzene or toluene, in a salt water solution down to concentrations of the order of 0.1 milligram per liter, or less, is now a routine determination using a chromatograph column. Such a method, for example, is outlined in Mc- Auliife, C., Solubility in Water of Paraffin, Olefin, Acetylene, Cycloolefin and Aromatic Hydrocarbons, Amer= ican Chemical Society, 148th Meeting, Abstracts, p. 17 (1964). Accordingly, the matter of obtaining the concentration of, for example benzene or toluene, in the sample of formation water from the well formation is simply a matter of care in obtaining an uncontaminated sample and in transporting it in its initial state to the point where the hydrocarbon analysis is made.

On the other hand, the idea of use of a target value has not to my knowledge been previously considered. Experience has shown that the major cause for change in the concentration of benzene or toluene in a water solution immediately in contact with crude oil containing such aromatic hydrocarbons is the total dissolved solids present in the water. The nature of these solids is not particularly significant. Thus, whether the formation water contains essentially all sodium chloride or various other soluble sodium, potassium and calcium salts is of little consequence. The greater the total dissolved solids in the water, the lower will be the maximum concentration of the aromatic hydrocarbon constituents from any particular crude oil.

This is illustrated by the graph of FIG. 1 in which the aromatic hydrocarbon solubility has been plotted against the total salinity of the water in contact with the oil. Curve 11 shows the concentration of benzene in waters of varying salinity in immediate contact with a tertiary oil from Louisiana. Curve 12 shows the equivalent curve for toluene. Curves 13 and 14 show the aromatic hydrocarbon solubility of benzene and toluene, respectively, for waters of varying salinity in contact with an oil found in a Silurian formation in Montana. Curves 15 and 16, in turn, give the same data for Mississippian formation oil found in Montana. In each case the experimental technique employed was to prepare a number of solutions of varying known salt content which were contacted for a minimum of six days with samples of crude oil from the same formation. A sample of each water solution was then run through the chromatograph column to determine the concentratoon of at least one aromatic hydrocarbon, i.e., benzene or toluene (or both). The resultant aromatic hydrocarbon solubility was then plotted against the corresponding salinity. In all cases, data for a single oil at varying values of salinity in the water contacting it gave rise to essentially a straight line on semilog paper, as the curves in FIG. 1 show. Of course, the absolute values to be expected from any particular crude oil would be expected to vary, depending upon the oil-soluble hydrocarbon concentration in the crude oil. It was found that essentially all curves for one type of aromatic hydrocarbon (for example, benzene) were essentially parallel, as is shown by curves 11, 13 and 15, or by 12, 14 and 16 (for toluene). It is possible by this technique, therefore, to compile a library of aromatic hydrocarbon solubility versus salinity for any crude oil which could be expected to occur in a particular reservoir at a particular geographic location.

Accordingly, if a well drills through the Silurian reservoir rock in Montana, and if an uncontaminated sample of the formation water from this formation shows a salinity of grams per liter, it is simply necessary to enter FIG. 1 at the value of salinity determined and draw a vertical line 17 up to the point a at which the curve for the appropriate hydrocarbon is intersected. The corresponding ordinate in this case for benzene (curve 13) is about 1.7 milligrams per liter. This is the target value for water of the specified salinity in contact with that particular type of crude oil. Thus, if the hydrocarbon concentration of benzene in the sample of formation water from the Silurian reservoir showed a value of, say, 1.6 milligrams per liter, it would be apparent that the well has been drilled quite close to the oil-contact; Where as if the value were 0.4 milligram per liter under the same circumstances, the well would be removed some distance (usually under four miles, as indicated above) from the edge of the oil reservoir.

In the use of this method it is necessary to take certain precautions in obtaining and transmitting the sample of water obtained in the test. In the first place, it is of course important to avoid contamination in obtaining the sample. Contamination may be due to a lack of isolation of the zone from other zones in the well, or from the presence of hydrocarbons in the drilling fluids, etc. Contamination is minimized if the drill stem test is made with the zone properly isolated mechanically with packers. Contamination from drilling fiuid already in the test zone is minimized if an adequate volume of water is recovered. Thus, for example, a drill stem formation fluid column of 1,000 feet or greater generally provides the best samples. How ever, somewhat smaller amounts of recovered formation fluids have been used. It is also desirable if an oil base drilling mud is used, or if oil is incorporated in the drilling mud, to use diesel oil instead of crude oil. Crude oil is the only known natural source of benzene and toluene.

Each sample should be caught in a metal sample con tainer, preferably of around 1 quart capacity, or in a. clean bucket, and immediately transferred to a metal sample container. The container should be filled completely to the top, then the sides of the can should be lightly squeezed to allow for fluid expansion while the lid is sealed on tightly. A foil-lined cap should be provided. Experience has shown that if water (or oil for that matter) is collected to the top in a metal container with a cap that is tightly secured, any benzene or toluene concentration will remain relatively constant at least four or five weeks. FIG. 2 shows comparative data for aromatic hydrocarbon concentration as measured in containers which were full and tightly capped, as compared to those which were full but not tightly capped, con= tainers partly full though tightly capped, and containers partly full but not tightly capped. It is apparent that an erroneous value can be obtained if these simple precautions concerning the container are not followed.

Preferably, more than one sample should be obtained from each drill stem test. The best procedure involves taking a sample from near the top of the fluid column recovered, near the middle, and near the bottom of the column. These are of course separately collected and transmitted. Normally the top and middle recovery samples are analyzed for chloride concentration, for example, and compared with the concentration from the bottom sample as an aid in evaluating contamination of the bottom sample. This bottom sample is usually the least contaminated. If the chloride analyses of the middle and bottom samples are fairly similar compared to that of the top sample, the bottom sample probably has not been seriously contaminated.

It was earlier mentioned that the aromatic hydrocarbon solubility is primarily a function of the total dissolved solids in the formation water. It is apparent, therefore, that it is not actually necessary to determine the total dissolved solids, but any quantity directly related to the total salinity of the sample can be equivalently employed. Thus, for example, it is possible to measure the total chloride content of the sample and employ this as the salinity in connection with FIG. 1. One can similarly determine the specific gravity of the water sample and compare it to the comparable specific gravities corresponding to the salinities plotted across the bottom of the graph in FIG. 1. Again, since the electric resistivity of? the sample is directly related to the salinity, it is possible to measure the resistivity of the formation water and compare it to a curve like FIG. 1, in which the aromatic hydrocarbon solubility is determined as a function of the electric resistivity of a plurality of samples of varying salt concentration, directly equivalent to the method dis cussed in detail above.

Basically, the improvement which has been shown in this specification is that of obtaining a target value for any water sample, the aromatic hydrocarbon solubility of which is to be determined, by contacting samples of the expected crude oil against water solutions of varying salt concentration to determine the hydrocarbon solu= bility as a function of salinity or equivalent so that the maximum aromatic hydrocarbon solubility to be expected, if the formation water were in immediate contact with this crude oil, can be determined,

1 claim:

1. A. method of geochemical prospecting for crude oil of a type known to occur in the region under investigation comprising the steps of (1) contacting said type of crude oil with water solutions of salt of at least two different concentrations for a sufiicient time to permit the concentration of at least one aromatic hydrocarbon in said solutions to reach at least substantially equilibrium value,

(2) determining the concentration of said at least one aromatic hydrocarbon in each of said solutions, whereby the variation in said concentration may be determined as a function of the salinity of the dissolving water,

(3) taking a sample of formation Water from a well penetrating the formation under investigation,

(4) determining a quantity directly related to the salinity of said sample, from which a target value of said at least one aromatic hydrocarbon can be established for said sample from the results of step (2), and

(5) measuring the concentration of said at least one aromatic hydrocarbon in said sample.

2. The method of claim 1 in which said aromatic hydrocarbon is chosen from the class consisting of ben-= zene and toluene.

3. The method of claim 2 in which said quantity directly related to the salinity of said sample is the total chloride content of said sample.

4. The method of claim 2 in which said quantity directly related to the salinity of said sample is the specific gravity of said sample.

5. The method of claim 2 in which said quantity directly related to the salinity of said sample is the total dissolved solids of said sample.

6. The method of claim 2 in which said quantity directly related to the salinity of said sample is the electric resistivity of said sample.

7. The method of claim 2 in which a plurality of formation water samples are taken and said quantity is determined for each of said samples, to permit use of that quantity representing the least contamination of the samples by extraneous fluids.

8. The method of claim 2 in which said sample of formation water is maintained in a metal container, com= pletely filled and sealed against leakage to the atmosphere, between the time said sample is obtained at the surface above said well and that at which said sample is analyzed for concentration of said at least one aromatic hydrocarbon.

References Cited UNITED STATES PATENTS 2,767,320 10/ 1956 Coggeshall eeeeee =2 23-230 X 3,033,287 5/1962 Bond u 166250 X 3,428,431 2/1969 Billings .a 23-230 RICHARD C. QUEISSER, Primary Examiner C. E. SNEE HI, Assistant Examiner US. Cl. XR. 

